The present invention relates to an apparatus for manufacturing flat panel display devices composed of thin film transistors formed in such a manner that a semiconductor film formed on an insulating substrate is irradiated with a laser beam to attain improvement of film quality or magnification or pseudo single crystallization of crystal grains.
At present, a liquid crystal display device or an organic electroluminescence (EL) display device forms an image by switching of thin film transistors formed from an amorphous silicon film on a substrate such as a glass substrate or a fused quartz substrate. If a driver circuit for driving the pixel transistors can be formed together with the transistors on the substrate, there is some expectation that reduction in production cost and improvement in reliability will be made remarkably.
In the present situation, the crystallinity of a silicon thin film used for forming active layers of such transistors is however poor, so that the performance of the thin film transistors represented by mobility becomes low. It is difficult to produce circuits satisfying a high speed and a high function. To produce such high-speed high-function circuits, high-mobility thin film transistors are required. To achieve this, it is necessary to improve the crystallinity of the silicon thin film.
Excimer laser annealing has been heretofore used as a method for improving the crystallinity. This method intends to improve mobility by applying an excimer laser beam on an amorphous silicon film (with mobility of 1 cm2/Vs or less) formed on an insulating substrate such as a glass substrate to transform the amorphous silicon film into a polycrystalline silicon film. The crystal grain size of the polycrystalline film obtained by excimer laser irradiation is the order of hundreds of nm. The mobility of the polycrystalline film is about 150 cm2/Vs. The performance of the polycrystalline film is sufficient to drive pixel thin film transistors but insufficient to be applied to thin film transistors for forming high-speed operating circuits such as driver circuits for driving a display panel. Incidentally, the thin film transistors may be hereinafter referred to as “transistors” simply.
Protrusions with a size of from the order of tens of nm to the order to hundreds of nm are formed in grain boundaries. The protrusions cause reduction in transistor with stand voltage. Moreover, the process margin of the excimer laser beam is narrow because the pulse energy of the excimer laser beam varies widely. Moreover, there are disadvantages as follows. That is, the equipment cost for excimer laser irradiation is high because toxic gas must be used. In addition, the running cost for excimer laser irradiation is very high because an expensive oscillator tube must be exchanged periodically.
As a method for solving these problems, a method in which a silicon thin film is irradiated with a laser beam in such a manner that the second harmonic of a pulse-duration-controlled continuous-wave (CW) solid-state laser modulated by an electro-optical (EO) modulator is condensed linearly while the silicon thin film is scanned with the laser beam has been disclosed in Patent Document 1. This method intends to increase the crystal grain size by elongating the melting duration of silicon and accelerating reduction in cooling rate.
In the aforementioned background art, mobility higher than 500 cm2/Vs is obtained in such a manner that an amorphous silicon thin film formed on a glass substrate is scanned with the second harmonic of a laser diode (LD)-pumped CW solid-state laser beam to grow crystals in the scanning direction of the laser beam. The polycrystalline film obtained thus has no protrusion. If the silicon thin film having this degree of mobility is obtained, driver circuits of sufficient performance can be formed, so that a so-called “system-on” panel can be achieved.    [Patent Document 1] Japanese Patent Laid-Open No. 2003-124136    [Patent Document 2] Japanese Patent Laid-Open No. 2003-53578    [Patent Document 3] Japanese Patent Laid-Open No. 283933/1999